Mol 12A (IRAS 05373+2349): a very young and deeply embedded high-mass Herbig Be star

ABSTRACT Mol 12A (IRAS05373+2349) is a Class I high-mass young stellar object located at a distance of 1.59 kpc and still embedded ($A_V \simeq 12$) in its natal core. Its bolometric luminosity is $1.74 \times 10^3\, {\rm L}_\odot$. Medium-resolution spectra that, combined, cover 0.6–2.4 $\mu$m are presented. These show a large number of atomic permitted lines in emission, mostly hydrogen, C i, N i, Ca i, Ca ii, Si i, Fe i, Fe ii, and He i. The lines originate in the inner zones of an active protoplanetary disc. Notably, the He i 1.0830 $\mu$m line shows a complex P-Cygni profile, indicating the presence of strong stellar and/or inner disc winds accompanied with high accretion. From the strength of the brightest lines, we estimate the accretion luminosity and accretion rate to be $L_{\rm acc} \sim 10^3 \,{\rm L}_\odot$ and $\dot{M} \sim 10^{-5}\, {\rm M}_\odot$ y$^{-1}$. First overtone CO emission band-heads are detected at 2.3 $\mu$m, confirming the high accretion. No photospheric absorption lines are present. Multicolour photometric monitoring of Mol 12A from 2016 to 2024 is reported, based mostly on the Zwicky Transient Facility data together with additional targeted observations. The light-curves appear complex: at $\lambda < 2.5~\mu$m, Mol 12A exhibits irregular brightness dips, amounting to more than 4 mag at 1.25 $\mu$m accompanied by colour changes that suggest the occurrence of eclipses of up to $A_V \simeq 25$ of extra extinction caused by dust structures, probably fragments of the disrupted disc. The out-of-eclipse de-reddened location of Mol 12A on the HR-diagram and PARSEC isochrones suggest stellar parameters $M = 7 {\!-\! }9\, {\rm M}_\odot$, $T_{\rm eff} = 16\,000 {\!-\!} 19\,000\, {\rm K}$, $R = 3 {\!-\!} 6\, {\rm R}_\odot$, and age $\simeq 0.5$ Myr.